Add handling of special default values

[bennothommo]

The Builder plugin currently cannot handle certain valid default values for certain datatypes when creating a new table in the Database panel. A suitable PR for this issue would adjust the migration generator to allow these certain values in the "Default" column:

• All fields with nullable ticked: Should accept null as a valid value to be rendered as null in the migration. Should also accept "null" (with quotes) as a string value of null for the (very rare) case that a dev needs null stored as a string in a field.
• Boolean fields: Should accept true and false as valid values to be rendered as true and false respectively.